Substrates for printed circuit boards are widely used, the substrates including metal layers formed of, for example, metals stacked on surfaces of insulating resin films formed of, for example, resins and being configured to provide printed circuit boards by etching the metal layers to form conductive patterns.
There has been a need for a substrate for a printed circuit board, the substrate having a high adhesive force between a resin film and a metal layer so that when a folding force is applied to a printed circuit board formed by using such a substrate for a printed circuit board, the metal layer does not peel off from the resin film.
In recent years, with the miniaturization of electronic devices, a decrease in the thicknesses of printed circuit boards has also been required. In view of this, various methods for directly stacking a metal layer on a surface of a resin film without using an adhesive have been proposed. For example, a technique has been proposed in which a metal layer is formed on a surface of a resin film by reducing a metal ion in a state where the resin film is immersed in a solution containing the metal ion (refer to Japanese Unexamined Patent Application Publication No. 2013-161928). This patent application publication discloses that, as an index of adhesiveness between a resin film and a metal layer, a peel strength between the resin film and the metal layer after a weather resistance test including holding at 150° C. for seven days should be increased.